The combination of magic-angle spinning and dipolar decoupling for resolution enchancement, with cross- and double-cross polarization for sensitivity and selectivity enhancement, has made practical the characterization of the structure and dynamics of single sites of proteins in biological solids. We propose to develop three additional new solids NMR experiments to help characterize protein binding sites: (1) rotational-echo double resonance, (2) DANTE-differenced 13C-13C spin diffusion, and (3) combined DANTE- and double-cross polarization spin transfer. These experiments will measure quantitatively dipolar coupling between 13C, 15N, 31P and 17O in both covalent and ionic bonds. The latter will extend solids NMR techniques to protein binding sites in stable charge-neutralized complexes. We propose to apply solids NMR experiments to the identification and characterization of stable-isotope single-, double-, and triple- labeled binding sites of immobilized proteins, protein adducts, and the peptide stems of peptidoglycans in intact bacterial cell walls.